'server closed' in singleplayer

I created a superflat world where there was only one layer and it was sand. a village spawned and caused a block update. I placed a block below me and faced about 5 min of ~0 fps. After framerate had returned i was poking around the now floating village when suddenly the world froze. a few seconds later a box saying "server closed" was in the middle of the screen
NOTE: I was in singleplayer